Influence of the Crystal Surface on the Austenitic and Martensitic Phase Transition in Pure Iron

Using classical molecular dynamics simulations, we studied the influence that free surfaces exert on the austenitic and martensitic phase transition in iron. For several single-indexed surfaces—such as ${\left(100\right)}_{\mathrm{bcc}}$ and ${\left(110\right)}_{\mathrm{bcc}}$ as well as ${\left(100\right)}_{\mathrm{fcc}}$ and ${\left(110\right)}_{\mathrm{fcc}}$ surfaces—appropriate pathways exist that allow for the transformation of the surface structure. These are the Bain, Mao, Pitsch, and Kurdjumov–Sachs pathways, respectively. Tilted surfaces follow the pathway of the neighboring single-indexed plane. The austenitic transformation temperature follows the dependence of the specific surface energy of the native bcc phase; here, the new phase nucleates at the surface. In contrast, the martensitic transformation temperature steadily decreases when tilting the surface from the (100) ${}_{\mathrm{fcc}}$ to the (110) ${}_{\mathrm{fcc}}$ orientation. This dependence is caused by the strong out-of-plane deformation that (110) ${}_{\mathrm{fcc}}$ facets experience under the transformation; here, the new phase also nucleates in the bulk rather than at the surface.